1. Field of the Invention
The present invention relates to a vehicle generator control system for controlling output voltage of a vehicle AC generator to be mounted in a passenger car or a truck.
2. Description of the Related Art
In a conventional generator control system for controlling output voltage of a vehicle AC generator, field current is supplied to a generator when a key switch sensor detects turn-on of a key switch. However, it is necessary to provide signal wires for detecting the operation of the key switch.
In another generator control system such as disclosed in JP-U-62-44698, field current is supplied when rotation of a rotor of a vehicle AC generator is detected by a magnetic sensor that detects residual magnetic flux of the rotor. This can omit special signal wires. However, the electromotive force generated by the residual magnetic flux is so small that it is difficult to detect rotation of the rotor accurately. The residual magnetic flux becomes weaker if the AC generator is not operated for a comparatively long time, for example, when a vehicle is shipped abroad.
U.S. Pat. Nos. 5,182,511 and 5,602,470 disclose detector circuits that detect a voltage difference between two phase-windings of a vehicle AC generator, thereby detecting rotation of an engine.
However, those disclosed generator control systems necessitate a special detector circuit, which is a normally closed circuit as shown in FIG. 10. Therefore, it is necessary to provide a protection circuit to prevent a large amount of current from flowing into the detector circuit when the AC generator starts generation. This also makes the voltage control system complicated and expensive.
Therefore, an object of the present invention is to provide a simple and improved vehicle generator control system that does not require any special wire or circuit for detecting operation of a key switch.
According to a feature of the invention, a generator control system of a vehicle AC generator includes switching means connected in series with a field coil, a voltage regulator for controlling the switching means to thereby control the output voltage of the vehicle AC generator, comparing means for providing a pulse signal by comparing voltage of a phase-winding with voltage of another phase-winding and engine start detecting means for starting operation of a voltage regulator when frequencies of the pulse signal becomes larger than a predetermined value.
Therefore, no closed circuit is formed, so that a large amount of current can be prevented from flowing into the engine start detecting means without providing any protection circuit. In addition, only a low level of voltage difference between two phase-windings is necessary to provide a pulse signal without using a special signal wire.
Preferably, the two phase-windings are 90 degree in electric angle different in phase from each other, in order to detect the engine start accurately.
It is also preferable for the generator control system to include bypassing means for connecting the two phase-windings to a ground if the output voltage of the phase-windings is larger than a threshold value.
As shown in FIG. 11, if leak current flows into a phase-winding Y, the voltage Vpy that is inputted to an engine start detecting circuit drifts from the phase voltage Py of the phase-winding Y by Vbxc3x97R2/(R1+R2), where Vb is terminal voltage of a battery B, R1 is a contact resistance of a portion to which the leak current flows, and R2 is an internal resistance of the engine start detecting circuit. The bypassing means reduces the drift voltage, as shown in FIG. 8.
It is also preferable for generator control system to include means for supplying field current for a predetermined period if the output voltage of one of the two phase-windings becomes higher than a second threshold voltage that is lower than the first threshold voltage.
It is possible to discriminate normal output voltage of one of the phase-windings from drifted output voltage that is caused by by leak current.